1. Field of the Invention
The present invention relates to base station-to-handset interfaces for radio telephone systems, and is useful in particular, but not exclusively, in PCS (personal communications systems) utilizing cable television plants as signal conduits.
The invention may also be employed, for example, in telephone systems utilizing dedicated coaxial cable and/or fiber optic and/or microwave signal conduits.
2. Description of the Related Art
It is expected that PCS microcells will soon be supporting a rapidly increasing number of mobile handsets utilizing time division duplex protocols in North America. To support this user base, it is essential that the PCS-microcells be both low power to assist frequency re-use, and low cost, because the net capital costs of the PCS-microcells will be a major factor in the economic viability of PCS.
It has been suggested by a number of organizations that existing cable television distribution plant be used to interconnect microcell equipment. Taking advantage of the broadband and the nearly ubiquitous nature of cable TV plant, it has been further proposed that the microcell equipment consist of simple RF repeaters that simply translate off-air mobile voice traffic onto the cable plant and vice versa.
It has become apparent in tests that this approach to PCS-microcells yields both low capital costs and improved user service.
In summary, the low cost arises from the combination of simple technology (an RF repeater), using an existing asset base (i.e. cable plant), in a fashion that allows modulation/demodulation and PSTN interface equipment to be centrally located. This allows these equipment costs to be amortised over a very large net coverage area.
The improved service arises from better call blocking probability associated with the ability to centralize the base station equipment rather than a proper allocation to specific microcells. Additionally, the cable plant can act to form distributed antenna arrays that can be shaped into "roamer corridors". Within these roamer corridors it is also possible to control the radio signal dynamic range so as to reduce near user/far user interactions and line of sight blocking.
The cable TV plant supports frequency division duplex (FDD) protocols, but not time division duplex (TDD) protocols. Consequently, prior art systems employ remote antenna drivers (RADs) for effecting FDD-to-TDD conversions and TDD-to-FDD conversions at the microcell serving the TDD handsets, and a remote antenna signal processor (RASP) for effecting the TDD-to-FDD and FDD-to-TDD conversions at a central location.
It is, however, a disadvantage of such a system that a significant time delay occurs in the transmission of the different signals between the base station and the handset, because of propagation delays over the cable TV plant. The handset synchronizes with a local remote antenna driver, as explained in greater detail below, but the base station perceives the receive signals from the handset as arriving late, by approximately twice the above-mentioned delay.